Preheat your oven to 400°F (200°C).

In a skillet over medium heat, melt the butter. Add the thinly sliced onions and cook for 15-20 minutes until deep golden brown and caramelized.

In a large bowl, toss the cauliflower florets with olive oil, salt, and pepper. Spread the florets on a baking sheet in a single layer.

Roast the cauliflower in the preheated oven for about 25 minutes or until tender and lightly golden.

Combine the roasted cauliflower with caramelized onions in a baking dish. Sprinkle the grated Gruyère cheese over the top and return to the oven for an additional 10-15 minutes until melted and bubbly.